X-ray point-projection radiography using an X pinch as the x-ray source appears to be a very powerful tool for studying plasma objects like exploding wires and X pinches. In such experiments, the material atomic numbers are relatively high (Z>12) and line densities (the total mass along the ray path) are large enough to see significant x-ray absorption. But in some tests, very small amounts of low Z materials, such as the residual plastic insulation around an exploded wire, have been clearly visible in images. The explanation of this phenomenon is that wave optics, rather than just the usual ray optics, must be taken into account in the radiographic image analysis. If the source of radiation is highly coherent and the film is placed far enough behind the object being radiographed, a fringe pattern is seen that results from the interference of different parts of the x-ray beam that are diffracted and refracted by sample structures. A combination of absorption, refraction and diffraction effects, involving interference fringes at the edges of sample features, will be visible on the film. The first observations of such fringe patterns with x-ray beams were made with x-ray radiation with energies of 10-50 keV from a synchrotron and with x-rays from a microfocus x-ray tube with a voltage of 40-60 keV. As is known, spatial coherence (or small effective size of the source) is much more important than spectral coherence.
X-ray radiation with energies higher than 1-20 keV is useful for imaging strongly absorbing objects, such as several cm scale or larger life forms, thick tissue samples, parts in manufacturing processes, and samples of interest in material science. In order to study small biological objects, such as insects, tissue samples from 1 mm up to a few cm, and plants up to a few cm thick, softer radiation (E=1-20 keV) is required.
The 1-20 keV spectral band is impractical to access using small, table-top size devices like x-ray tubes and laser plasmas. Microfocus x-ray tubes have very low efficiency in the wavelength band <10 keV, resulting in low radiation intensity and long exposure time. For a laser plasma to reach this spectral band, extremely high power density is required, and it is practically impossible to achieve a source size smaller than 10-15 microns without paying a large efficiency penalty by using a tiny pinhole. Synchrotrons are an excellent source of 5-20 keV radiation, but they are extremely expensive and not available everywhere.
Previous disclosures have investigated the use of X-pinch as an X-ray source for micro-lithography and other uses but have failed to teach the use of X-pinch as a source of high resolution phase contrast radiography.
For example, U.S. Pat. No. 5,102,775 issued to Hammer et al, discloses a method and apparatus for x-ray microlithography using an X-pinch having a source size of a few tens of microns, formed by the crossing of two wires constructed out of aluminum or magnesium creating a photon having an energy of up to 1.6-1.84 KeV. However, the taught wire materials, energy levels and resulting source sizes are generally insufficient for providing phase-contrast resolution less than 3 microns, and appear incapable of producing a resolution of approximately 1 micron or less as is desirable for high resolution phase contract radiography.